According to the MongoDB Atlas documentation, the M0 free tier has a limit of 500 concurrent incoming connections.
With regard to your question about the amount of data that you can save with 512 MB of space, is “it depends.” MongoDB has a maximum document size limit of 16 MB, which means if you maxed out your documents, you could save 512 MB/16MB = 32 documents. However, this is VERY unusual, and most users have MUCH smaller documents.
For example, I have a M0 cluster and each document is about 500 bytes, which means I can save about 512 Mb (4096000000 bytes) / 500 bytes = 8,192,000 Documents before I max out my M0 tier, This example tends to be more of an “average” users experience, but again, it depends on your data schema.
You can find more information about limits of each tier here.